TITLE: GCN CIRCULAR NUMBER: 23366 SUBJECT: GRB 181023A: Swift detection of a burst DATE: 18/10/23 02:21:45 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT F. E. Marshall (NASA/GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), M. J. Moss (George Washington University) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:04:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181023A (trigger=868427). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 248.329, +19.564 which is RA(J2000) = 16h 33m 19s Dec(J2000) = +19d 33' 51" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 02:07:17.8 UT, 195.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 248.32979, 19.57140 which is equivalent to: RA(J2000) = 16h 33m 19.15s Dec(J2000) = +19d 34' 17.0" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 26 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (6.04 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.4 (+2.82/-2.40) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 199 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)